Substrate for magnetic disk and magnetic disk

ABSTRACT

A magnetic-disk glass substrate has a circular center hole a pair of main surfaces and an edge surface. The edge surface has a side wall surface and chamfered surfaces interposed between the side wall surface and the main surfaces, and a roundness of an edge surface on an outer circumferential side is 1.5 μm or less. Also, a midpoint A between centers of two least square circle respectively derived from outlines in a circumferential direction respectively obtained at two positions spaced apart by 200 μm in a substrate thickness direction on the side wall surface on the outer circumferential side, and centers B and C respectively derived from a respective one of two chamfered surfaces on the outer circumferential side in the substrate thickness direction, are located such that a sum of respective distances between A and B, and A and C, is 1 μm or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 14/770,025,filed on Aug. 24, 2015, now U.S. Pat. No. 9,595,286, which is a U.S.National stage application of International Patent Application No.PCT/JP2014/055114, filed on Feb. 28, 2014, which, in turn, claimspriority under 35 U.S.C. § 119(a) to Japanese Patent Application No.2013-041303, filed in Japan on Mar. 1, 2013, the entire contents ofwhich are hereby incorporated herein by reference.

BACKGROUND

Field of the Invention

The present invention relates to a magnetic-disk glass substrate and amagnetic disk.

Background Information

Nowadays, personal computers, digital versatile disc (DVD) recorders,and the like have a built-in hard disk drive (HDD) for data recording.In particular, in a hard disk drive that is used in a device premised onportability such as a notebook-type personal computer, a magnetic diskin which a magnetic layer is provided on a glass substrate is used andmagnetic recording information is recorded to or read from the magneticlayer with a magnetic head that flies slightly above the surface of themagnetic disk. A glass substrate is unlikely to be plastically deformedcompared with a metal substrate (aluminum substrate) or the like, andthus is preferably used as the substrate of this magnetic disk.

Moreover, the density of magnetic recording has been increased to meetthe demand for an increase in the storage capacity of hard disk drives.For example, the magnetic recording information area has been madesmaller using a perpendicular magnetic recording system that causes thedirection of magnetization in the magnetic layer to be perpendicular tothe surface of the substrate. This makes it possible to increase thestorage capacity per disk substrate. In such a disk substrate, it ispreferable that the substrate surface is made as flat as possible andthe direction in which magnetic particles grow is arranged in thevertical direction such that the direction of magnetization in themagnetic layer faces in a substantially perpendicular direction relativeto the substrate surface.

Also, in order to further increase the storage capacity, by using amagnetic head equipped with a dynamic flying height (DFH) mechanism tomake the flying height of the magnetic head from the magnetic recordingsurface extremely short, the magnetic spacing between the recording andreproducing element of the magnetic head and the magnetic recordinglayer of the magnetic disk is reduced, thus further improving theaccuracy of the recording and reproduction of information (improving theS/N ratio). Also in this case, it is required to make the surfaceunevenness of a magnetic-disk substrate as small as possible in orderfor the magnetic head to stably read/write magnetic recordinginformation over a long period of time.

Servo information that is used to position the magnetic head at a datatrack is recorded on the magnetic disk. It is conventionally known thatwhen the roundness of an edge surface of the magnetic disk on the outercircumferential side (also referred to as “outer circumferential edgesurface” hereinafter) is reduced, the magnetic head flies stably, andthus the servo information is favorably read, and the magnetic headstably reads/writes information. For example, the technique described inJP 2008-217918A discloses a magnetic-disk glass substrate in which theroundness of the outer circumferential edge surface is 4 μm or less.With this glass substrate, the durability against load/unload (LUL)testing is improved by reducing the roundness of the outercircumferential edge surface.

SUMMARY

Incidentally, in recent years, HDDs using a shingle write system inwhich recording is performed such that adjacent tracks partially overlapwith each other are known. With the shingle write system, signaldeterioration caused by recording to an adjacent track is extremelysmall, thus making it possible to dramatically increase the trackrecording density and to achieve an extremely high track recordingdensity of 500 kTPI (tracks per inch) or more, for example. On the otherhand, due to the increase in TPI, the tracking performance of themagnetic head for servo signals is more rigorously required than before.

In a HDD having a track recording density of 500 kTPI or more using theshingle write system, for example, a phenomenon where the servo signalswas unstably read occurred at the edge portion of the magnetic disk onthe outer circumferential side even when the roundness of the outercircumferential edge surface of the magnetic disk was reduced to 1.5 μmor less. It is conceivable that this phenomenon is caused by stablereading being affected by the magnetic disk vibrating (referred to asfluttering) due to disturbance of air flow on the outermostcircumferential side of the outer circumferential side edge portion ofthe magnetic disk. The outer circumferential side edge portion of a mainsurface of the magnetic disk is more likely to be influenced by fluttercompared to a region more on the inner circumferential side, and thus itis difficult to perform reading stably.

An object of the present invention is to provide a magnetic-disk glasssubstrate and a magnetic disk that are capable of suppressingdisturbance of air flow near the outer circumferential side edge portionof the magnetic disk and suppressing flutter.

In order to suppress disturbance of air flow near the outercircumferential side edge portion of a magnetic disk and to eliminateinfluences resulting from clamping the magnetic disk, the inventorassembled a HDD by eliminating play (a gap between the inner hole of themagnetic disk and a spindle) to precisely align the center of themagnetic disk with the center of the spindle. Accordingly, wobble of theouter circumferential edge surface of the magnetic disk in the diskradial direction was made less than or equal to the roundness of theouter circumferential edge surface such that the influence of theroundness of the edge surface of the magnetic disk on the innercircumferential side, and the influence of the concentricity of theinner circumferential edge surface and the outer circumferential edgesurface were eliminated, but flutter was not reduced.

It was conventionally thought that flutter is suppressed when theroundness of the magnetic disk is reduced, and that roundness andflutter are correlated. However, according to studies conducted by theinventor of the present invention, flutter was not suppressed even whenthe roundness was reduced to 1.5 μm or less, and it was revealed that ina case where the roundness was extremely small, roundness and flutterwere not correlated.

It was thought that the reason for this was as follows. That is, theroundness of the outer circumferential edge portion has beenconventionally measured by positioning a plate-shaped probe that islonger than the thickness of a glass substrate vertically with respectto the main surface of the glass substrate and bringing the probe intocontact with the outer circumferential edge portion. At this time, theprobe is in contact with a position of the substrate that projects mostoutwardly in the substrate thickness direction. Accordingly, the shapeof the substrate that projects most outwardly is reflected to theoutline of the outer circumferential edge portion that serves as a basisof the roundness measurement, irrespective of the shape of the outercircumferential edge portion in the substrate thickness direction.Therefore, with the conventional method for measuring roundness, thethree-dimensional shape of the side wall surface of the outercircumferential edge portion in the substrate thickness direction wasnot reflected. In the case where the outer circumferential edge portionof the magnetic disk was provided with sufficiently favorable roundnessbased on the conventional method for measuring the roundness, it wasthought that the influence that factors other than roundness exerted onflutter was relatively increased, and thus that the correlation betweenroundness and flutter was no longer evident.

In view of this, the inventor of the present invention focused attentionon the shape of the magnetic disk in the substrate thickness directionin addition to parameters of the magnetic disk in a direction parallelto the main surface of the magnetic disk such as roundness, and firststudied variation in substrate thickness at the outer circumferentialside edge portion of the magnetic disk, but the variation was extremelysmall, and no problems could be found. In view of this, it was revealedthat the inclination and unevenness of a side wall surface (a surfaceextending in a direction orthogonal to the main surface) of the outercircumferential edge surface of the magnetic disk or chamfered surfaces(surfaces interposed between the side wall surface and the mainsurfaces) affected flutter in the outermost circumferential portion ofthe magnetic disk. That is, it was revealed that by making the roundnessof the outer circumferential edge surface of the magnetic disk extremelysmall, the shape of the outer circumferential edge surface in thesubstrate thickness direction affected flutter.

As a result of further studies, it was found that a distance between thecentral axis of the side wall surface of the magnetic disk on the outercircumferential side and the center of the two chamfered surfacesgreatly influenced flutter. In other words, it was found that if thisdistance is large, flutter is likely to increase. In the case where themagnetic disk is viewed as a structure having a first cylinder havingone axis, and second and third cylinders of smaller diameter that arelocated on both sides in the axis direction, it can be thought that thisdistance corresponds to the size of the displacement of the axes ofthese three cylinders. It is thought that flutter changes due to achange in eccentricity resulting from this axis displacement.

A magnetic-disk glass substrate of the present invention is amagnetic-disk glass substrate having a circular hole at a center, andincluding a pair of main surfaces and an edge surface,

the edge surface having a side wall surface and chamfered surfacesinterposed between the side wall surface and the main surfaces,

a roundness of the edge surface on an outer circumferential side being1.5 μm or less, and

when outlines in a circumferential direction are respectively obtainedat two positions spaced apart by 200 μm in a substrate thicknessdirection on the side wall surface on the outer circumferential side,and a midpoint between centers of two least square circles respectivelyderived from the outlines is given as a midpoint A, and

when outlines in the circumferential direction are respectively obtainedat positions that are located at centers of the two chamfered surfaceson the outer circumferential side in the substrate thickness direction,and among centers of least square circles derived from the outlines, acenter derived from one chamfered surface is given as a center B, and acenter derived from the other chamfered surface is given as a center C,

a sum of a distance between the midpoint A and the center B and adistance between the midpoint A and the center C being 1 μm or less.

In the magnetic-disk glass substrate of the present invention,preferably the sum is 0.5 μm or less. In the magnetic-disk glasssubstrate of the present invention, in a case where, with regard to asurface roughness of the side wall surface on the outer circumferentialside, a maximum height in the substrate thickness direction is Rz(t) anda maximum height in the circumferential direction is Rz(c), preferablyRz(t)/Rz(c) is 1.2 or less.

In the magnetic-disk glass substrate of the present invention, when ameasurement point is provided every 30 degrees in the circumferentialdirection, referenced on the center of the glass substrate, and a radiusof curvature of a shape of a portion between the side wall surface andthe chamfered surface on the outer circumferential side at themeasurement point is derived, preferably a difference in the radius ofcurvature between adjacent measurement points is 0.01 mm or less.

In the magnetic-disk glass substrate of the present invention,preferably outlines of the side wall surface in the circumferentialdirection are respectively obtained at a plurality of differentpositions in the substrate thickness direction, including at least threepositions spaced apart by 100 μm in the substrate thickness direction onthe side wall surface on the outer circumferential side, an inscribedcircle and a circumscribed circle of each outline are obtained, and adifference in the radius between a smallest inscribed circle and alargest circumscribed circle is 5 μm or less.

The magnetic-disk glass substrate of the present invention is preferablyused in a case where a substrate thickness is 0.5 mm or less. A magneticdisk of the present invention is a magnetic disk in which a magneticlayer is formed on the magnetic-disk glass substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of a magnetic-disk glass substrate of anembodiment;

FIG. 1B is a cross-sectional view in a substrate thickness direction ofthe magnetic-disk glass substrate of the embodiment;

FIG. 2 is a diagram illustrating a method for measuring a shapeevaluation value of an outer circumferential edge surface of themagnetic-disk glass substrate of the embodiment;

FIG. 3 is a diagram illustrating a method for measuring the shapeevaluation value of the outer circumferential edge surface of themagnetic-disk glass substrate of the embodiment;

FIG. 4 is a diagram illustrating a method for measuring a cylindricityof a side wall surface of the magnetic-disk glass substrate of theembodiment;

FIG. 5 is a diagram illustrating a method for measuring the cylindricityof the side wall surface of the magnetic-disk glass substrate of theembodiment; and

FIG. 6 is an enlarged view of a portion of a cross-section of the outercircumferential side of the magnetic-disk glass substrate of theembodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a magnetic-disk glass substrate of this embodiment will bedescribed in detail.

[Magnetic-Disk Glass Substrate]

Aluminosilicate glass, soda-lime glass, borosilicate glass, or the likecan be used as a material for a magnetic-disk glass substrate of thisembodiment. In particular, aluminosilicate glass can be preferably usedbecause it can be chemically strengthened and be used to produce amagnetic-disk glass substrate having excellent flatness of its mainsurfaces and excellent strength of the substrate. Amorphousaluminosilicate glass is preferable since smoothness of the surface,such as roughness, can be improved.

Although there is no limitation on the composition of the magnetic-diskglass substrate of this embodiment, the glass substrate of thisembodiment is preferably amorphous aluminosilicate glass having acomposition including, in terms of oxide amount in mol %, SiO₂ in anamount of 50 to 75%, Al₂O₃ in an amount of 1 to 15%, at least onecomponent selected from Li₂O, Na₂O, and K₂O in a total amount of 5 to35%, at least one component selected from MgO, CaO, SrO, BaO, and ZnO ina total amount of 0 to 20%, and at least one component selected fromZrO₂, TiO₂, La₂O₃, Y₂O₃, Ta₂O₅, Nb₂O₅, and HfO₂ in a total amount of 0to 10%.

The glass substrate of this embodiment may be preferably amorphousaluminosilicate glass having a composition including, in mass %, SiO₂ inan amount of 57 to 75%, Al₂O₃ in an amount of 5 to 20% (it should benoted that the total amount of SiO₂ and Al₂O₃ is 74% or more), ZrO₂,HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃, and TiO₂ in a total amount of more than0% to 6% or less, Li₂O in an amount of more than 1% to 9% or less, Na₂Oin an amount of 5 to 28% (it should be noted that a mass ratio Li₂O/Na₂Ois 0.5 or less), K₂O in an amount of 0 to 6%, MgO in an amount of 0 to4%, CaO in an amount of more than 0% to 5% or less (it should be notedthat the total amount of MgO and CaO is 5% or less and the content ofCaO is larger than that of MgO), and SrO+BaO in an amount of 0 to 3%,for example.

The glass substrate of this embodiment may also be crystallized glasscontaining, in terms of oxide amount in mass %, SiO₂ in an amount of45.60 to 60%, Al₂O₃ in an amount of 7 to 20%, B₂O₃ in an amount of 1.00to less than 8%, P₂O₅ in an amount of 0.50 to 7%, TiO₂ in an amount of 1to 15%, and RO (it should be noted that R represents Zn and Mg) in atotal amount of 5 to 35%, CaO in an amount of 3.00% or less, BaO in anamount of 4% or less, no PbO component, no As₂O₃ component, no Sb₂O₃component, no component, no NO⁻ component, no SO²⁻ component, no F⁻component, and one or more selected from RAl₂O₄ and R₂TiO₄ (it should benoted that R represents one or more selected from Zn and Mg) as a maincrystal phase, in which the particle size of crystals in the maincrystal phase is in a range of 0.5 nm to 20 nm, the degree ofcrystallization is 15% or less, and the specific gravity is 2.95 orless, for example.

The composition of the magnetic-disk glass substrate of this embodimentmay include SiO₂, Li₂O and Na₂O, and one or more alkaline earth metaloxides selected from the group consisting of MgO, CaO, SrO and BaO asessential components, the molar ratio of the content of CaO to the totalcontent of MgO, CaO, SrO, and BaO (CaO/(MgO+CaO+SrO+BaO)) may be 0.20 orless, and the glass-transition temperature may be 650° C. or higher. Themagnetic-disk glass substrate having such a composition is preferablefor a magnetic-disk glass substrate to be used in a magnetic disk forenergy-assisted magnetic recording.

The magnetic-disk glass substrate of this embodiment is a thin annularglass substrate. Although there is no limitation on the size of themagnetic-disk glass substrate, the magnetic-disk glass substrate ispreferable as a magnetic-disk glass substrate having a nominal diameterof 2.5 inches or 3.5 inches, for example. It should be noted that thethickness (0.635 mm, 0.8 mm, 1 mm, 1.27 mm, or the like) of themagnetic-disk glass substrate referred to in the description below is anominal value and actual measurement values may be slightly larger orsmaller than the nominal values.

FIGS. 1A and 1B show a magnetic-disk glass substrate G of thisembodiment. FIG. 1A is a plan view of the magnetic-disk glass substrateG and FIG. 1B is a cross-sectional view of the magnetic-disk glasssubstrate G in the substrate thickness direction.

The magnetic-disk glass substrate G (also referred to as “glasssubstrate G” as appropriate hereinafter) has a circular hole in thecenter, a pair of main surfaces 11 p and 12 p, and an edge surface. Theedge surface has a side wall surface 11 w, and chamfered surfaces 11 cand 12 c interposed between the side wall surface 11 w and the mainsurfaces 11 p and 12 p. In the magnetic-disk glass substrate G of thisembodiment, the outer circumferential edge surface has a roundness of1.5 μm or less and a shape evaluation value (described later) of 1 μm orless.

The method for measuring roundness can be a known method. For example,as described above, a plate-shaped probe that is longer than thethickness of the glass substrate is arranged vertically with respect tothe main surface of the glass substrate so as to be opposed to the outercircumferential edge surface, an outline is obtained by rotating theglass substrate in the circumferential direction, and thus a differencein the radius between an inscribed circle and a circumscribed circle ofthis outline can be calculated as the roundness of the glass substrate.It should be noted that a roundness/cylindrical shape measurement devicecan be used to measure roundness, for example.

The shape evaluation value of the glass substrate G will be describedwith reference to FIGS. 2 and 3. FIGS. 2 and 3 are diagrams illustratinga method for measuring the shape evaluation value of the outercircumferential edge surface of the magnetic-disk glass substrate G ofthis embodiment. FIG. 2 shows the cross-section of the outercircumferential edge surface of the glass substrate G in the substratethickness direction. There is no particular limitation on theinclination angle of the side wall surface 11 w, and the inclinationangle is 40 degrees to 70 degrees, for example. In addition, theboundaries between the side wall surface 11 w and the chamfered surfaces11 c and 12 c are not limited to a shape having an edge as shown in thediagram, and may have a smoothly continuous curved shape.

Outlines in the circumferential direction are respectively obtained attwo positions 37 and 38 spaced apart by 200 μm in the substratethickness direction on the side wall surface 11 w, and the midpointbetween centers 37 o and 38 o of two least square circles 37 c and 38 cthat are respectively derived from these outlines is given as a“midpoint A”. Furthermore, outlines in the circumferential direction arerespectively obtained at positions 34 and 35 that are located at centersof the two chamfered surfaces 11 c and 12 c in the substrate thicknessdirection, and among centers 34 o and 35 o of least square circles 34 cand 35 c that are respectively derived from these outlines, one center34 o derived from the chamfered surface 11 c is given as a “center B”,and the other center 35 o derived from the chamfered surface 12 c isgiven as a “center C”. In this case, the shape evaluation value is thesum of a distance a between the midpoint A and the center B and adistance b between the midpoint A and the center C. The shape evaluationvalue of the glass substrate G is preferably 1.0 μm or less, and morepreferably 0.5 μm or less.

The two positions 37 and 38 on the side wall surface 11 w are positionsthat are respectively spaced apart by 100 μm from the central positionof the glass substrate G in the substrate thickness direction toward themain surface 11 p side and the main surface 12 p side, for example. Themeasurement positions 34 and 35 for obtaining the outlines of thechamfered surfaces 11 c and 12 c are positions that are respectivelycloser to the central position side in the substrate thickness directionby an equal distance from the main surfaces 11 p and 12 p, for example(positions that are respectively closer to the central position by 0.075mm from the main surfaces 11 p and 12 p of the glass substrate G in acase where the chamfered surfaces of the glass substrate G have a lengthof 0.15 mm in the substrate thickness direction, for example).

As a measurement device for measuring the shapes of the outercircumferential edge surface at the measurement positions 37, 38, 34 and35, the roundness/cylindrical shape measurement device can be used, forexample. A stylus 3 of the roundness/cylindrical shape measurementdevice can move in micron units in the vertical direction (substratethickness direction).

It should be noted that the thickness of the glass substrate G ismeasured in advance with a micrometer prior to the measurement.Moreover, an outline shape measurement device is used to measure theshape, the length in the substrate thickness direction, the length inthe radial direction, and the inclination angle with respect to the mainsurface of the chamfered surface in a cross-section in the radialdirection, and in addition, the length of the side wall surface inadvance. The position of the boundary between the chamfered surface andthe side wall surface can be determined as an intersection point of anextension line of the side wall surface and an extension line of thechamfered surface in the case where both the side wall surface and thechamfered surface have a linear outline. In the case where the chamferedsurface and the side wall surface have an arc outline, each outline isapproximated by a circle that best overlaps the outline, and theposition of the boundary can be determined as an intersection point ofthe derived circles. In the case where the outline of the chamferedsurface or the side wall surface has both a linear portion and an arcportion, it is sufficient that the position of the boundary isdetermined by combining the above-described methods as appropriate.

At the time of measurement, the glass substrate G is set in theroundness/cylindrical shape measurement device such that the mainsurface of the glass substrate G is horizontal with respect to areference plane of the measurement device, and in addition, the centerof the glass substrate G coincides with a rotation center of themeasurement device. The height of a point at the front end of the stylus3 that comes into contact with the glass substrate G in the measurementis matched with the height of the upper main surface of the glasssubstrate G that has been set in the measurement device. When the stylus3 is lowered by a half distance of the substrate thickness in thesubstrate thickness direction in this state, the stylus 3 is disposed ata height of the center of the glass substrate G in the substratethickness direction. Then, outlines of the outer circumferential edgeportion of the glass substrate G are measured at the point 37 to whichthe stylus 3 is raised by 100 μm from the center of the substratethickness and the point 38 to which the stylus 3 is lowered by 100 μmfrom the center of the substrate thickness. The centers 37 o and 38 o ofthe two least square circles 37 c and 38 c of the side wall surface 11 ware determined from these outlines, and in addition, the midpoint Abetween the two centers 37 o and 38 o is determined.

In addition, the position of the stylus 3 is set to an intermediateheight of each of the two chamfered surfaces in the substrate thicknessdirection, and outlines of the outer circumferential edge portion of theglass substrate G are measured at the positions 34 and 35. The centers Band C of the least square circles 34 c and 35 c of the chamferedsurfaces 11 c and 12 c are determined based on these outlines. Next, thedistance a between the midpoint A and the center B, and the distance bbetween the midpoint A and the center C are summed, and thus the shapeevaluation value is derived.

It should be noted that in a structure that has the above-describedthree cylinders having different diameters, it is conceivable that thepositions 34 and 35, which are respectively at intermediate heights ofthe chamfered surfaces in the substrate thickness direction, are pointsthat best express the degree of eccentricity of the cylinderscorresponding to the chamfered surfaces. In addition, it is thought thatthese positions are points that most affect air flows near the chamferedsurfaces. For these reasons, it is preferable to measure the outlines atthese positions. The shape evaluation value derived from the side wallsurface 11 w and the chamfered surfaces 11 c and 12 c is adjusted bychamfering processing with a formed grindstone, edge surface grindingprocessing, and brushing, which will be described later, for example.

In the glass substrate G of this embodiment, it is preferable that theside wall surface 11 w has a cylindricity of 5 μm or less. By settingthe cylindricity of the side wall surface 11 w to 5 μm or less, air flowbetween the inner wall of the HDD and the side wall surface is unlikelyto be disturbed and flutter is further suppressed, leading to areduction in the number of servo errors.

The cylindricity of the glass substrate G will be described withreference to FIGS. 4 and 5. FIGS. 4 and 5 are diagrams illustrating amethod for measuring the cylindricity of the side wall surface on theouter circumferential side of the magnetic-disk glass substrate G ofthis embodiment. Outlines 31 a, 32 a, and 33 a of the side wall surface11 w in the circumferential direction are respectively obtained at aplurality of different positions in the substrate thickness directionincluding at least three positions 31, 32, and 33 spaced apart by 100 μmin the substrate thickness direction on the side wall surface 11 w, andinscribed circles and circumscribed circles of the respective outlinesare obtained, with “cylindricity” referring to a difference R betweenthe radius of a smallest inscribed circle C1 and the radius of a largestinscribed circle C2. It should be noted that it can be said that thecloser to zero such an evaluation index, that is, the radius differenceR, is, the closer to a geometric cylinder the shape of the outercircumferential edge surface is, and therefore, the evaluation index isreferred to as “cylindricity” in this description. FIG. 5 is a diagramillustrating a method for measuring the cylindricity of an outercircumferential edge surface of the glass substrate G.

There are three measurement positions on the side wall surface 11 w inthis embodiment. Among the three measurement positions 31, 32, and 33,the measurement position 32 is the central position of the glasssubstrate G in the substrate thickness direction, for example. Themeasurement positions 31 and 33 are spaced apart from the measurementposition 32 by 100 μm in the substrate thickness direction, for example.It should be noted that the measurement positions 31 and 32 are providedat positions spaced apart from the measurement position 32 by 100 μm inthe substrate thickness direction in the case where the magnetic-diskglass substrate has a substrate thickness of 0.635 mm. In the case wherethe substrate thickness is different, the distance from the measurementposition 32 to the measurement positions 31 and 32 respectively in thesubstrate thickness direction may be changed. For example, in the caseof a magnetic-disk glass substrate having a substrate thickness of T(mm), the distance may be set to 100 (μm)×(L/0.635).

As a measurement device for measuring the shapes of the outercircumferential edge surface of the glass substrate G at the measurementpositions 31 to 33, a device is used that can obtain outlines 31 a, 32 aand 33 a distinctly from each other at the measurement positions 31 to33 of the side wall surface 11 w. From this point, it is preferable thatthe stylus 3 of the measurement device has a spherical surface having aradius of curvature of 0.4 mm or less. At the time of measurement, thestylus 3 is disposed so as to be opposed to each of the measurementpositions 31 to 33 on the side wall surface 11 w of the glass substrateG, and performs the measurement on those positions in succession.

The respective outlines 31 a to 33 a of the measurement positions 31 to33 are obtained by rotating the glass substrate G by one cycle in astate in which the stylus 3 is disposed so as to be opposed to themeasurement positions 31 to 33. An inscribed circle and a circumscribedcircle are respectively obtained based on the center derived by theleast squares method with respect to the three obtained outlines 31 a to33 a, and a circumscribed circle C2 that is in contact with theoutermost side and an inscribed circle C1 that is in contact with theinnermost side are determined. The radius difference R between thecircumscribed circle C2 and the inscribed circle C1 is derived as thecylindricity of the side wall surface 11 w. The cylindricity of the sidewall surface 11 w is adjusted by chamfering processing with a formedgrindstone, edge surface grinding processing, and brushing, for example.

With regard to the surface roughness of the side wall surface 11 w onthe outer circumferential side, in the case where the maximum height inthe substrate thickness direction is Rz(t) and the maximum height in thecircumferential direction is Rz(c), Rz(t)/Rz(c) is preferably 1.2 orless, and more preferably 1.1 or less. If Rz(t)/Rz(c) exceeds theabove-described range, there are cases in which variation in the shapeevaluation value of substrates is likely to increase at the time of massproduction. By setting Rz(t)/Rz(c) to a value in the above-describedrange, it is possible to reduce variation in the shape evaluation value.

It should be noted that the value of surface roughness can be obtainedby measuring the side wall surface 11 w with a wavelength bandwidth inwhich the surface roughness is measured using a laser microscope setfrom 0.25 μm to 80 μm, for example, and selecting and analyzing a regionof 50 μm square in the measured range. The surface roughness in thesubstrate thickness direction and the circumferential direction can takean average value of data obtained by measuring the line roughness of theregion of 50 μm square, for example, from a plurality of cross-sectionsrespectively corresponding in the substrate thickness direction and thecircumferential direction. For example, it is sufficient that five setsof data are obtained and the average thereof is used as the surfaceroughness.

The surface roughness of the side wall surface 11 w on the outercircumferential side preferably has a maximum height Rz of 0.2 μm orless and more preferably 0.1 μm or less. Also, the surface roughnessthereof preferably has an arithmetic mean roughness Ra of 0.02 μm orless. Setting Rz and Ra in this range can prevent thermal asperityresulting from adherence or digging in of foreign substances andcorrosion resulting from the deposition of ions such as sodium andpotassium. Also, for similar reasons to those described above, it isalso preferable that the surface roughness of the pair of chamferedsurfaces 11 c and 12 c is in the above-described range. Here, theabove-described Rz refers to the maximum height defined by JIS B0601:2001. Ra refers to the arithmetic mean roughness defined by JIS B0601:2001.

In this embodiment, when a measurement point is provided every 30degrees in the circumferential direction, referenced on the center ofthe glass substrate G, and a radius of curvature of a shape of a portionbetween the side wall surface 11 w and the chamfered surfaces 11 c and12 c at the measurement point is derived, it is preferable that adifference in the radius of curvature between adjacent measurementpoints is set to 0.01 mm or less. The number of measurement points is12. Accordingly, it is possible to reduce changes in the shape of theouter circumferential edge surface in the circumferential direction ofthe magnetic-disk glass substrate G, and reduce variation in the shapeevaluation value of the outer circumferential edge portion. It should benoted that the difference in the radius of curvature between adjacentmeasurement points is more preferably 0.005 mm or less, becausevariation in the shape evaluation value of the outer circumferentialedge portion can be further reduced.

Referring to FIG. 6, the radius of curvature of the shape of a portionbetween the side wall surface 11 w and the chamfered surface 11 c isderived as follows, for example. FIG. 6 is an enlarged view of a portionof an outer circumferential cross-section of the magnetic-disk glasssubstrate G of this embodiment.

First, on the cross-section of the glass substrate in the substratethickness direction at one measurement point, an intersection point of afirst virtual line L1 obtained by extending a linear portion of thechamfered surface 11 c and a second virtual line L2 obtained byextending a linear portion of the side wall surface 11 w is given as a“first intersection point P1”. Next, a third virtual line L3 passingthrough the first intersection point P1 and extending perpendicular tothe linear portion of the chamfered surface 11 c is set. Next, anintersection point of the third virtual line L3 and the portion betweenthe side wall surface 11 w and the chamfered surface 11 c is given as a“second intersection point P2”. Also, a first circle C3 having apredetermined radius (50 μm, for example) around the second intersectionpoint P2 is set on the cross-section of the magnetic-disk glasssubstrate G. Also, two intersection points of an outer circumference ofthe first circle C3 and the portion between the side wall surface 11 wand the chamfered surface 11 c are respectively given as a “thirdintersection point P3” and a “fourth intersection point P4”.Furthermore, a second circle C4 respectively passing through the second,third, and fourth intersection points P2, P3, and P4 are set. Byderiving the radius R of the second circle C4, the radius of curvatureof the shape of the portion between the side wall surface 11 w and thechamfered surface 11 c can then be derived.

It should be noted that the radii of curvature of shapes of bothportions between the side wall surface and the chamfered surfaceadjacent to one main surface and between the side wall surface and thechamfered surface adjacent to the other main surface can also be derivedin the manner described above.

The magnetic-disk glass substrate G described above has an extremelysmall roundness and shape evaluation value. Therefore, disturbance ofair flow is unlikely to occur at the outer circumferential side edgeportion, thus suppressing flutter. This makes it possible to retain thetracking performance for servo information at the outer circumferentialside edge portion. In particular, the tracking performance for servoinformation is more rigorously required in a disk having a high trackrecording density, such as a magnetic disk to which a shingle writesystem is applied, and this glass substrate G can be favorably used inthe magnetic disk.

It is thought that the reason why flutter is suppressed due to a smallshape evaluation value is as follows. If the outer circumferential edgeportion of the glass substrate G has a large roundness, the amount ofair pushed in the horizontal direction (radial direction) by the outercircumferential edge surface of the magnetic disk varies, causing alarge disturbance of air flow. However, if the outer circumferentialedge surface has an extremely small roundness, such a large disturbanceof air flow is unlikely to occur. In a state in which the outercircumferential edge surface has an extremely small roundness, ratherthan air flow in the horizontal direction, it is the smoothness withwhich air flows in the substrate thickness direction through the gapbetween the outer circumferential edge portion of the glass substrate Gand the inner wall of the HDD so as to span the magnetic disk that isimportant.

With the studies conducted by the inventor of the present invention, itwas found that in a HDD, the air steadily flows in the substratethickness direction through the gap between the inner wall of the HDDand the outer circumferential edge surface of the magnetic disk, and ifa phenomenon that disturbs the flow and makes it irregular occurs, thelevel of flutter increases and the magnetic head flies unstably.Conversely, if the outer circumferential edge surface of the glasssubstrate G has a small shape evaluation value, the air steadily flowssmoothly in the substrate thickness direction through the gap betweenthe inner wall of the HDD and the outer circumferential edge surface ofthe magnetic disk, and thus the level of flutter is unlikely toincrease.

As described above, in a HDD having an extremely high track recordingdensity, the disturbance of air flow in the HDD is an important factorin improving the tracking performance of the magnetic head for servoinformation. Such disturbance of the air causes flutter to increase.There are two types of air disturbance, namely, disturbance that occursperiodically (steadily) and disturbance that occurs unexpectedly. Ofthese, the disturbance that occurs periodically can often be eliminatedby changing the design of the HDD, but the disturbance that occursunexpectedly cannot be suppressed by changing the design of the HDD, andtherefore, it is required to suppress this disturbance using anothermeans. The inventor of the present invention found that the outercircumferential edge surface of the glass substrate G caused disturbanceof air flow that could not be eliminated by changing the design of theHDD, and thus achieved the glass substrate G in which the outercircumferential edge surface had an extremely small shape evaluationvalue.

The glass substrate G of this embodiment has a substrate thickness of0.8 mm, 0.635 mm, or 0.5 mm or less, for example. In the case where theglass substrate G is used in a magnetic disk, the magnetic disk is morelikely to rattle and flutter is more likely to increase as the glasssubstrate G becomes thinner. However, the glass substrate G has theshape evaluation value of 1 μm or less as described above, andtherefore, in the case where the glass substrate G is used in a magneticdisk, disturbance of air flow is suppressed at the outer circumferentialside edge portion, and flutter is suppressed.

Furthermore, it is preferable that the glass substrate G of thisembodiment has an extremely small shape evaluation value and the outercircumferential edge surface thereof has a shape that is unlikely tocause disturbance of air flow. If the shape evaluation value is smaller,in the case where the glass substrate G is used in a magnetic disk,flutter can be further suppressed. This makes the tracking performanceof the magnetic head for servo information in the HDD more favorable.

When a magnetic disk on which a magnetic layer having a track recordingdensity of 500 kTPI (tracks per inch) or more, in particular, is formed,such as a magnetic disk for a shingle write system or a magnetic diskfor energy-assisted magnetic recording, is incorporated in the HDD, thetracking performance of the magnetic head of the HDD for servoinformation may be deteriorated in the case where the magnetic diskflutters, and therefore, the magnetic-disk glass substrate of thisembodiment is preferable for the magnetic disk having a high recordingdensity described above.

It is preferable that the glass substrate G of this embodiment has a duboff value, which is an evaluation index of the outer circumferentialedge portion on the main surface, of 30 nm or less. In addition, it ispreferable that the dub off value is greater than zero. When an outlinebetween two points that are a point at a radius of 31.2 mm and a pointat a radius of 32.2 mm is measured in an outline of the main surface ofthe glass substrate G in the radial direction and the two points areconnected by a virtual straight line, the dub off value refers to amaximum distance from the virtual straight line to the outline of themain surface of the glass substrate G. It should be noted that when thevirtual straight line is compared with the outline of the main surfaceand the virtual straight line is located on the center side in thesubstrate thickness direction, the dub off value is positive.Conversely, when the outline of the main surface is located on thecenter side in the substrate thickness direction, the dub off value isnegative. The closer to zero this value is, the flatter and morefavorable the shape of the main surface near the outermost circumferenceis, and therefore, the magnetic head flies stably. Accordingly, this incombination with an extremely small roundness and shape evaluation valuecan suppress disturbance of air flow at the outer circumferential edgeportion of the substrate, reduce variation in flutter, and improve theyield of HDDs at the time of mass production. The dub off value can bemeasured using an optical surface shape measurement device, for example.It should be noted that the dub off value in this description isobtained by measuring a region on the outer circumferential side withrespect to the conventional measurement range. This makes it possible toevaluate a difference in the shape of the edge portion with higheraccuracy than before.

It is preferable that in the glass substrate G of this embodiment, themain surface on the outer circumferential side edge portion of the mainsurface has a nanowaviness (NW-Rq) of 0.5 Å or less. Here, thenanowaviness can be expressed by an RMS (Rq) value calculated asroughness having a wavelength bandwidth of 50 to 200 μm in an annularregion of a radius of 30.5 to 31.5 mm, and can be measured using anoptical surface shape measurement device, for example. This incombination with an extremely small roundness and cylindricity cansuppress disturbance of air flow at the outer circumferential edgeportion of the substrate, reduce variation in flutter, and improve theyield of HDDs at the time of mass production.

[Method for Manufacturing Magnetic-Disk Glass Substrate]

Hereinafter, a method for manufacturing a magnetic-disk glass substrateof this embodiment will be described step-by-step. It should be notedthat the order of the steps may be changed as appropriate.

(1) Glass Substrate Formation

A raw glass plate is molded by press molding and processing isappropriately performed to form an inner hole and an outer shape toobtain a disk-shaped glass substrate having the inner hole having apredetermined substrate thickness, for example. It should be noted thatthe method for molding a raw glass substrate is not limited to thesemethods and a glass substrate can also be manufactured by a knownmanufacturing method such as a float method, a down draw method, aredraw method, or a fusion method.

(2) Edge Surface Grinding Step

Next, the edge surfaces of the annular glass substrate are ground. Theedge surfaces of the glass substrate are ground in order to formchamfered surfaces at the outer circumferential side edge portion andthe inner circumferential side edge portion of the glass substrate, andadjust the outer and inner diameters of the glass substrate. Thegrinding processing performed on the outer circumferential side edgesurface of the glass substrate may be known chamfering processing with aformed grindstone using diamond abrasive particles, for example.

The outer circumferential side edge surface of the glass substrate ofthis embodiment is ground using a formed grindstone and by additionalgrinding processing in which a grindstone is brought into contact withthe edge surface of the glass substrate such that a locus of thegrindstone, which is in contact with the edge surface of the glasssubstrate, is not constant. Hereinafter, the additional grindingprocessing on the outer circumferential side edge surface of the glasssubstrate will be described.

A grindstone used to additionally grind the outer circumferential sideedge surface of the glass substrate G is formed in a cylindrical shapeas a whole and has a groove. The groove is formed so as to be capable ofsimultaneously grinding both the side wall surface 11 w and thechamfered surface 11 c of the glass substrate G on the outercircumferential side. Specifically, the groove has a groove shapeincluding a side wall portion and chamfering portions located on bothsides of the side wall portion. The side wall portion and the chamferingportions of the groove described above are formed so as to havepredetermined dimensions and shapes in consideration of the finishingtarget dimensions and shapes of the ground surfaces of the glasssubstrate G.

In the processing of the outer circumferential side edge surface of theglass substrate, the grinding processing is performed by rotating boththe glass substrate G and the grindstone while bringing the grindstoneinto contact with the outer circumferential side edge surface of theglass substrate G in a state in which the glass substrate G is inclinedwith respect to the groove direction of the groove formed in thegrindstone, that is, in a state in which a rotation axis of the glasssubstrate G is inclined at an angle α with respect to a rotation axis ofthe grindstone. Accordingly, the locus of the grindstone that abutsagainst the outer circumferential side edge surface of the glasssubstrate G is not constant, and the abrasive particles of thegrindstone abut against and act on the edge surface of the substrate atrandom positions. Therefore, since impairment of the substrate isreduced, the surface roughness of the ground surface is reduced, andin-plane variation is reduced, it is possible to make the ground surfacesmoother, that is, to finish the ground surface with a quality of alevel that meets the requirement for higher quality. Furthermore, theeffect of improving the life of the grindstone is obtained.

Moreover, the grindstone and the glass substrate G are in contact witheach other in a state in which the groove of the grindstone and an outerdiameter arc of the glass substrate G are in contact with each other ina surface contact state, thus increasing a contact area between thegrindstone and the glass substrate G. Therefore, a contact length(cutting blade length) of the grindstone with respect to the glasssubstrate G is extended, thus making it possible to maintain thesharpness of the abrasive particles. Accordingly, stable grindingperformance can be secured even in the case where the grindingprocessing is performed using a grindstone with fine abrasive particlesthat is advantageous in terms of the quality of the ground surface, andthe favorable quality of the ground surface (mirror surface quality) canbe stably obtained by grinding processing mainly using a plastic mode.In addition, the sharpness of the grindstone is maintained and thegrinding performance for achieving the plastic mode is stably secured,thus making it possible to secure the favorable accuracy of dimensionsand shapes obtained by chamfering processing performed on the outercircumferential side edge surface of the glass substrate.

Although the inclination angle α of the glass substrate G with respectto the groove direction of the grindstone described above can be setarbitrarily, it is preferable that the inclination angle α is in a rangeof two to eight degrees in order to more favorably exhibit theoperations and effects described above. In particular, in order toimprove the surface quality of the polished glass substrate G after thegrinding and reduce the machining allowance for the outercircumferential side edge surface and the inner circumferential sideedge surface of the glass substrate G in the brushing, it is preferablethat the inclination angle α is large. It is preferable that thegrindstone used in the grinding processing is a grindstone obtained bybinding diamond abrasive particles with resin (resin bond grindstone).It is preferable to use a 2000# to 3000# diamond grindstone.

A preferable example of the circumferential speed of the grindstone is500 to 3000 m/minute, and the circumferential speed of the glasssubstrate G is about 1 to 30 m/minute. In addition, it is preferablethat the ratio (circumferential speed ratio) of the circumferentialspeed of the grindstone with respect to the circumferential speed of theglass substrate G is in a range of 50 to 300.

It should be noted that the above-described grinding step can be dividedinto two steps, and first grinding is performed in a state in which therotation axis of the glass substrate G is inclined at an angle α (α>0),as described above, second grinding is performed in a state in which therotation axis of the glass substrate G is inclined at an angle −α usinganother grindstone, and adjustment is performed such that the machiningallowance of the second grinding is smaller than the machining allowanceof the first grinding, as a result of which Rz(t)/Rz(c) can be 1.2 orless.

It is preferable that the hardness (referred to as “grindstone hardness”hereinafter) obtained by measuring a binder (resin) portion on thegrindstone surface of the above-described resin bond grindstone using aBerkovich indenter under conditions where an indentation load is 250 mNby a nanoindentation test method is in a range of 0.4 to 1.7 GPa. In thecase of the resin bond grindstone, the grindstone hardness is an indexthat is correlated with a bond strength between the diamond abrasiveparticles and the resin.

As a result of grinding the outer circumferential side edge surfaceusing resin bond grindstones having various characteristics andobserving processed quality of the edge surface of the glass substrate,the inventor of the present invention found that the bond strengthbetween the diamond abrasive particles and the resin in the resin bondgrindstone had a large influence on the shape evaluation value of theinner hole of the glass substrate subjected to the above-describedgrinding processing. That is, it was found that if the outercircumferential side edge surface was ground using a resin bondgrindstone having a grindstone hardness that is too high, the processingrate was favorable but the surface was likely to be blemished and theshape evaluation value of the outer circumference deteriorated, whereasif the outer circumferential side edge surface was ground using a resinbond grindstone having a grindstone hardness that is too low, the shapeevaluation value of the outer circumference was favorable but theprocessing rate decreased markedly. In other words, the shape evaluationvalue of the outer circumference of the glass substrate can be adjustedby changing the grindstone hardness. As a result, it was found that thegrindstone hardness was preferably in the above-described range. Bysetting the grindstone hardness in the above-described range, it ispossible to process the outer circumferential side edge surface afterbeing ground to a semi-mirror surface, and therefore, the machiningallowance can be reduced in a subsequent edge surface polishing step,thus making it possible to improve the shape accuracy of the edgeportion including the shape evaluation value of the outer circumferencewhile maintaining high surface quality.

A method for measuring grindstone hardness by a nanoindentation testmethod will be described. A load is applied at 1 nm/sec to a binderportion of the grindstone surface, which is the measurement target,using a Berkovich indenter having a quadrangular pyramidal tip, thepressure is increased to 250 mN and held for a predetermined time (10seconds, for example), and then a relationship between the load and thedisplacement when the pressure is reduced at an unloading rateequivalent to when the pressure was increased is obtained. A curveobtained here indicates dynamic hardness, which is a characteristiccloser to actual use conditions than evaluation of hardness, which is aconventional static hardness characteristic. Based on the result of theobtained curve of dynamic hardness characteristics, grindstone hardnesscan be obtained by the nanoindentation test method using Equation (1)below.H=F/Ac  (1)

where H is the hardness of the grindstone, F is a load, and Ac is anindentation area.

The above-described indentation area Ac is expressed by relationalexpressions (2) and (3) below.Ac=f(hc)∝24.5·hc ²  (2)hc=h max−ε·F/S  (3)

where hc is an indentation depth, h max is a depth at maximum load, hsis an indentation depth at the start of unloading, ho is an indentationdepth after unloading, E is a shape coefficient specific to the indenter(e.g.: in case of a Berkovich indenter=0.75), S is a proportionalitycoefficient of the load and displacement, and m is a slope (dF/dh).

(3) Edge Surface Polishing Step

Next, the edge surfaces of the annular glass substrate are polished. Theedge surfaces of the glass substrate are polished in order to improvethe properties of the outer circumferential side edge surface and theinner circumferential side edge surface (side wall surface and chamferedsurfaces) of the glass substrate. In the edge surface polishing step,the outer circumferential side edge surface and the innercircumferential side edge surface of the glass substrate are polished bybrushing. The machining allowance for the glass substrate in thebrushing is set such that the surfaces of the side wall surface 11 w,and the chamfered surfaces 11 c and 12 c are mirror-surfaces.

By performing the edge surface grinding and the edge surface polishingdescribed above, contamination by attached waste and the like andimpairment such as scratches on the edge surface of the glass substratecan be eliminated, thermal asperity and deposition of ions such assodium and potassium that causes corrosion can be prevented, and surfaceroughness and waviness can also be significantly reduced and the shapeevaluation value of the outer circumferential edge surface of the glasssubstrate can be reduced, thus making it possible to improve the shapeaccuracy of the outer circumferential edge portion.

(4) First Polishing (Main Surface Polishing) Step

After the main surface grinding step is appropriately performed asrequired, first polishing is performed on the ground main surfaces ofthe glass substrate. In the first polishing step, the main surfaces ofthe glass substrate are polished using a double-side polishing deviceprovided with a planetary gear mechanism. The double-side polishingdevice has an upper surface plate and a lower surface plate. Planarpolishing pads are attached to the upper surface of the lower surfaceplate and the bottom surface of the upper surface plate. One or moreglass substrates accommodated in a carrier are sandwiched between theupper surface plate and the lower surface plate, and the glass substrateand the surface plates are moved relative to each other by the planetarygear mechanism moving one or both of the upper surface plate and thelower surface plate while supplying loose abrasive particles includingan abrasive, so that both main surfaces of the glass substrate can bepolished.

During the relative motion described above, the upper surface plate ispressed against the glass substrate (that is, in a vertical direction)with a predetermined load, the polishing pads are pressed against theglass substrate, and a polishing liquid is supplied between the glasssubstrate and the polishing pads. The main surfaces of the glasssubstrate are polished by the abrasive included in this polishingliquid. Known abrasive particles such as cerium oxide, zirconium oxide,and silicon dioxide can be used as the abrasive, for example. It shouldbe noted that this step may be divided into a plurality of steps inwhich the type or size of the abrasive particles is changed.

(5) Chemical Strengthening Step

Furthermore, as required, the glass substrate that was subjected to thefirst polishing step may be chemically strengthened. A molten liquid ofmixed salts of potassium nitrate and sodium nitrate, for example, can beused as a chemical strengthening liquid. Chemical strengtheningprocessing is performed by immersing the glass substrate in the chemicalstrengthening liquid, for example. In this manner, by immersing theglass substrate in the chemical strengthening liquid, lithium ions andsodium ions in the surface layer of the glass substrate are respectivelyreplaced by sodium ions and potassium ions with a relatively large ionradius in the chemical strengthening liquid, and the glass substrate isstrengthened.

(6) Second Polishing (Final Polishing) Step

Next, second polishing is performed on the glass substrate. In thesecond polishing, a polishing device similar to that in the firstpolishing can be used, for example. At this time, the second polishingdiffers from the first polishing in the type and size of loose abrasiveparticles and the hardness of the resin polisher.

Microparticles of colloidal silica or the like suspended in a slurry,for example, are used as the loose abrasive particles that are used inthe second polishing. This makes it possible to further reduce thesurface roughness of the main surfaces of the glass substrate and toadjust the shape of the edge portion in a preferable range. Amagnetic-disk glass substrate can be obtained in this manner.

[Magnetic Disk]

A magnetic disk can be obtained as follows using the magnetic-disk glasssubstrate.

A magnetic disk has a configuration in which at least an adherent layer,a base layer, a magnetic layer (magnetic recording layer), a protectinglayer and a lubricant layer are laminated on the main surface of themagnetic-disk glass substrate (referred to as merely “substrate”hereinafter) in this order from the main surface side, for example.

For example, the substrate is introduced into a film deposition devicethat has been evacuated and the layers from the adherent layer to themagnetic layer are sequentially formed on the main surface of thesubstrate in an Ar atmosphere by a DC magnetron sputtering method. CrTican be used in the adherent layer and CrRu can be used in the baselayer, for example. A CoPt-based alloy can be used in the magneticlayer, for example. Also, a CoPt-based alloy or a FePt-based alloyhaving an L₁₀ ordered structure is formed as the magnetic layer forthermally assisted magnetic recording. After the film deposition asdescribed above, by forming the protecting layer using C₂H₄ by a CVDmethod, for example, and subsequently performing nitriding processingthat introduces nitrogen to the surface, a magnetic recording medium canbe formed. Thereafter, by coating the protecting layer withperfluoropolyether (PFPE) by a dip coat method, the lubricant layer canbe formed.

The produced magnetic disk is preferably incorporated in a magnetic-diskdrive (hard disk drive (HDD)) serving as a magnetic recording andreproduction device provided with a magnetic head equipped with adynamic flying height (DFH) control mechanism and a spindle for fixingthe magnetic disk.

Working Examples and Comparative Examples

In order to confirm the effect of the magnetic-disk glass substrate ofthis embodiment, 2.5-inch magnetic disks (having an outer diameter of 65mm, an inner diameter of 20 mm, and a substrate thickness of 0.635 mm,and an angle of the chamfered surface of 45 degrees with respect to themain surface) were produced using manufactured magnetic-disk glasssubstrates. It should be noted that the shape of the cross-section ofthe chamfered surface in the radial direction was linear, the angle withrespect to the main surface was 45 degrees, the length of the chamferedsurface in the substrate thickness direction was 0.15 mm, and the lengthof the chamfered surface in the radial direction was 0.15 mm. The glasscomposition of the produced magnetic-disk glass substrate was asfollows.

(Glass Composition)

Amorphous aluminosilicate glass was used that contained SiO₂ in anamount of 63 mol %, Al₂O₃ in an amount of 10 mol %, Li₂O in an amount of1 mol %, Na₂O in an amount of 6 mol %, MgO in an amount of 19 mol %, CaOin an amount of 0 mol %, SrO in an amount of 0 mol %, BaO in an amountof 0 mol %, and ZrO₂ in an amount of 1 mol %. It should be noted thatthe molar ratio of the content of CaO to the total content of MgO, CaO,SrO and BaO (CaO/(MgO+CaO+SrO+BaO)) was zero, and the glass-transitiontemperature was 703° C.

[Production of Magnetic-Disk Glass Substrates of Working Examples andComparative Examples]

The magnetic-disk glass substrates of working examples were produced byperforming the steps of the method for manufacturing a magnetic-diskglass substrate according to this embodiment in the given order. Here,the press molding method was used in molding of the glass substrate, andan inner hole and an outer shape were formed, and the substratethickness was adjusted using a known method.

In the edge surface grinding step, chamfering and side wall surfaceprocessing was performed on the inner circumferential side edge surfaceand the outer circumferential side edge surface of the glass substratewith a formed grindstone using diamond abrasive particles to formchamfered surfaces and a side wall surface. Furthermore, with regard tothe outer circumferential side edge surface of the glass substrate, byadding grinding processing in which the edge surface of the glasssubstrate was inclined and brought into contact with the grindstone suchthat the locus of the grindstone abutting against the edge surface ofthe glass substrate was not constant, surface quality was furtherimproved while increasing the shape accuracy of the chamfered surfacesand the side wall surface.

The additional grinding processing was performed on the outercircumferential side edge surface of the glass substrate using a resinbond grindstone with 2500# diamond abrasive particles under thefollowing grinding conditions. At that time, the inclination angle (αdescribed above) of the glass substrate with respect to the groovedirection of the grindstone was set to 5 degrees and other conditionswere adjusted as appropriate. At that time, glass substrates that havedifferent shape evaluation values of the outer circumferential edgesurface were produced by adjusting the inclination angle (α describedabove) and other factors (e.g., grit of the grindstone, andcircumferential speed of the grindstone or the glass substrate) in theabove-described range as appropriate. It should be noted that althoughin the case of a working example 1 in Table 1, α=5 degrees, by furtherincreasing the inclination angle, the surface quality is improved afterthe grinding and the machining allowance for brushing performedthereafter can be reduced, and thus it is possible to further improvethe shape evaluation value.

In the edge surface polishing step, the brushing was performed on theinner circumferential side edge surface and the outer circumferentialside edge surface of the glass substrate, using a slurry containingcerium oxide abrasive particles as polishing abrasive particles. Itshould be noted that the machining allowance for a chamfered surface inthe edge surface polishing step was adjusted in accordance with thesurface quality after the edge surface grinding step as appropriate.

Thereafter, grinding was performed on the main surface using a knownmethod, and then two-step polishing and chemical strengthening wereperformed thereon. A polishing liquid containing cerium oxide abrasiveparticles was used in the first polishing, and a polishing liquidcontaining colloidal silica polishing abrasive particles was used in thesecond polishing. The chemical strengthening was performed before thesecond polishing. The glass substrate on which polishing has beenperformed was cleaned using a known cleaning method as appropriate.

Through the above steps, the magnetic-disk glass substrates of theworking examples and comparative examples were produced as shown inTable 1.

The roundness of the side wall surface of the magnetic-disk glasssubstrate was measured by the above-described method. The shapeevaluation value was calculated as shown in FIG. 2. Specifically,outlines were obtained at positions on the side wall surface that werespaced apart upward and downward by 100 μm from the central position ofthe side wall surface in the substrate thickness direction and positionson the chamfered surface that were spaced apart by 75 μm from the upperand lower main surfaces in the central direction of the substratethickness, the centers of the least square circles of the outlines thatwere measured based on the two positions on the side wall surface weredetermined, a midpoint (A) and centers (B and C) of the least squarecircles of the outlines of the chamfered surfaces were viewed in thesubstrate thickness direction, a distance between A and B and a distancebetween A and C were derived, and a value obtained by adding thesedistances was used as the shape evaluation value of the outercircumferential edge portion. All measurements were performed using aroundness/cylindrical shape measurement device.

[Evaluation Method]

The magnetic-disk glass substrates were formed into films as describedabove to produce magnetic disks of working examples and comparativeexamples. Fluttering was evaluated by measuring flutteringcharacteristic values of the samples of the magnetic disks of theworking examples and the comparative examples using a laser Dopplervibrometer. In the measurement of the fluttering characteristic value, amagnetic disk was mounted on the spindle of a 2.5-inch type HDD androtated, and the main surface of the rotating magnetic disk wasirradiated with a laser beam from the laser Doppler vibrometer. Itshould be noted that the cover of the HDD was provided with a hole forlaser beam irradiation. Next, the laser Doppler vibrometer received thelaser beam reflected by the magnetic disk, and thus the amount ofvibration in the thickness direction of the magnetic disk was measuredas a fluttering characteristic value. At this time, the flutteringcharacteristic values were measured under the following conditions.

Environment for HDD and measurement system: The temperature was kept at25° C. in a constant temperature and humidity chamber.

Rotation rate of magnetic disk: 7200 rpm

Laser beam irradiation position: Position 31 mm apart from the center(1.5 mm apart from the outer circumferential edge) of a magnetic disk inthe radial direction

Minimum value of diameter of inner wall of disk-attaching portion in HDDhousing: 65.880 mm

[Evaluation Criterion]

As described below, the results of evaluation of the measured flutteringcharacteristic values were divided into four levels, Levels 1 to 4, indescending order of favorability (that is, in increasing order of thefluttering characteristic value). Levels 1 and 2 are acceptable forpractical purposes for a HDD of 500 kTPI.

Level 1: 20 nm or less

Level 2: more than 20 nm to 30 nm or less

Level 3: more than 30 nm to 40 nm or less

Level 4: more than 40 nm

TABLE 1 Shape evaluation Evaluation Roundness (μm) value (μm) resultComp. Ex. 2 1.7 1.0 Level 4 Comp. Ex. 1 1.5 1.3 Level 3 Work. Ex. 1 1.51.0 Level 2 Work. Ex. 2 1.0 0.8 Level 2 Work. Ex. 3 1.0 0.5 Level 1Work. Ex. 4 0.8 0.3 Level 1

It was confirmed from the evaluation results in Table 1 that if theroundness exceeded 1.5 μm (comparative example 2), flutteringcharacteristics were large and not acceptable. In addition, in the casewhere the shape evaluation value exceeded 1 μm (comparative example 1),even if the roundness was 1.5 μm or less, fluttering characteristicswere not favorable. On the other hand, in the case where the roundnesswas 1.5 μm or less and the shape evaluation value was 1 μm or less(working examples 1 to 4), fluttering characteristics were favorable. Itshould be noted that as shown in the working examples 3 and 4, in thecase where the shape evaluation value was 0.5 μm or less, it wasconfirmed that fluttering characteristics were particularly favorable.As shown in the working examples 1 to 4, it is conceivable that in thecase where fluttering characteristics were favorable, an error wasunlikely to occur when a magnetic signal was written to or read out fromthe magnetic disk of the HDD, and positioning accuracy using a servo ofthe HDD was favorable.

It should be noted that when a magnetic-disk glass substrate having aroundness of 1.7 μm and a shape evaluation value of 0.5 μm (comparativeexample 3) was prepared and fluttering characteristic values weremeasured using the glass substrate, the level was 4. According to this,it was found that even in the case where the shape evaluation value was1.0 μm or less and the roundness exceeded 1.5 μm, the level of flutterwas not favorable.

Next, ten magnetic-disk glass substrates of the working example 1described above and ten magnetic-disk glass substrates of the workingexamples 5 and 6 were produced, and Rz, Ra, an average value ofRz(t)/Rz(c), and variation in shape evaluation values were derived. Rzof each glass substrate was 0.2 μm or less. It should be noted that Raof each glass substrate was 0.02 μm or less. The magnetic-disk glasssubstrates of the working examples 5 and 6 were produced under theproduction conditions of the working example 1 except that only the edgesurface grinding step was different. Specifically, in the workingexamples 5 and 6, when the outer circumferential side edge surface wasground in the edge surface grinding step, the first grinding wasperformed such that the inclination angle (α described above) of theglass substrate with respect to the groove direction of the grindstonewas 5 degrees, the second grinding was then performed such that theinclination angle of the glass substrate was −5 degrees using anothergrindstone, and adjustment was performed such that the machiningallowance of the second grinding was smaller than the machiningallowance of the first grinding. The evaluation results of the workingexamples 1, 5, and 6 are shown in Table 2. In Table 2, the average valueof Rz(t)/Rz(c) is the average value of Rz(t)/Rz(c) of the tenmagnetic-disk glass substrates, and “variation in the shape evaluationvalue” is the difference between the maximum value and the minimum valueof shape evaluation values of the ten magnetic-disk glass substrates.

It could be found from Table 2 that Rz(t)/Rz(c) was 1.2 or less, andthus variation in the shape evaluation value decreased. Also, it couldbe found that if Rz(t)/Rz(c) was 1.1 or less, variation in the shapeevaluation value further decreased.

TABLE 2 Average value of Variation in shape Rz(t)/Rz(c) evaluation value(μm) Work. Ex. 1 1.29 0.17 Work. Ex. 5 1.16 0.10 Work. Ex. 6 1.08 0.07

Next, ten samples (working examples 7 and 8) were produced under theproduction conditions of the working example 1 except that the machiningallowance of the edge surface grinding was changed, and variation in theshape evaluation values of the working examples 7 and 8 was derived.Variation in the shape evaluation value is, similarly to that shown inTable 2, the difference between the maximum value and the minimum valueof shape evaluation values of the ten samples.

Also, with regard to the working examples 1, 7, and 8, the radius ofcurvature of the portion between the side wall surface and the chamferedsurface of the outer circumferential edge portion was derived. It shouldbe noted that the shape prepared in the grinding step is bettermaintained as the machining allowance of the edge surface polishingdecreases, and thus shape accuracy can be increased. In other words, thedifference in the radius of curvature can be reduced at adjacentmeasurement positions in the circumferential direction of the outercircumferential edge portion.

The radius of curvature of one glass substrate was derived as follows.Specifically, 24 points of the outer circumferential edge portion,namely 12 points on the front surface side and 12 points on the backsurface side, were measured in total. Then, a difference in the radiusof curvature between adjacent measurement points in the 12 points on thefront surface side (twelve sets of data) and a difference in the radiusof curvature between adjacent measurement points in the 12 points on theback surface side (twelve sets of data) were derived, and amongtwenty-four sets of data in total, the maximum value was used as themaximum value of the radius of curvature of the glass substrate.Examples of measurement data are shown in Table 3. In Table 3, the frontsurface and the back surface of the glass substrate, which was themeasurement target, are respectively referred to as “A surface” and “Bsurface”. Also, in Table 3, a difference in the radius of curvature when“0 to 30 degrees” means the absolute value of differences in the radiiof curvature between a measurement point at 0 degrees and a measurementpoint at 30 degrees, for example. Also, the position of the A surface at30 degrees on the back side corresponded to the position of the Bsurface at 30 degrees, for example.

When the maximum value of a difference in the radius of curvature wasderived with regard to the ten samples of the working examples 1, 7, and8, the ten samples of the working example 1 had a maximum value of 0.010mm or less, the ten samples of the working example 7 had a maximum valueof 0.005 mm or less, and the ten samples of the working example 8 had amaximum value of 0.012 mm or less. Examples of measurement data shown inTable 3 are data of one sample having the largest maximum value ofdifferences in the radii of curvature of the working examples.

With regard to the working examples 1, 7, and 8, Table 4 shows themaximum value of differences in the radii of curvature (same as thevalue indicated in Table 3; the maximum value of ten samples) andvariation in the shape evaluation values. It could be found from Table 4that by setting the maximum value of differences in radii of curvatureto 0.01 mm or less, variation in the shape evaluation values could besignificantly reduced.

TABLE 3 Difference in radius of curvature (mm) Work. Ex. 1 Work. Ex. 7Work. Ex. 8 A B A B A B surface surface surface surface surface surface0 to 30 0.005 0.006 0.002 0.003 0.003 0.011 degrees 30 to 60 0.004 0.0050.002 0.004 0.010 0.010 degrees 60 to 90 0.004 0.006 0.001 0.003 0.0090.001 degrees 90 to 120 0.005 0.009 0.004 0.002 0.011 0.004 degrees 120to 150 0.005 0.009 0.002 0.001 0.004 0.002 degrees 150 to 180 0.0080.010 0.003 0.002 0.004 0.003 degrees 180 to 210 0.006 0.009 0.001 0.0030.010 0.001 degrees 210 to 240 0.004 0.007 0.003 0.004 0.003 0.004degrees 240 to 270 0.005 0.008 0.002 0.005 0.001 0.002 degrees 270 to300 0.003 0.008 0.003 0.003 0.012 0.003 degrees 300 to 330 0.003 0.0070.005 0.001 0.009 0.005 degrees 330 to 360 0.003 0.006 0.005 0.004 0.0040.007 degrees Max. value of 0.010 0.005 0.012 differences

TABLE 4 Max. value of differences Variation in shape in radii ofcurvature (mm) evaluation value (μm) Work. Ex. 1 0.010 0.10 Work. Ex. 70.005 0.07 Work. Ex. 8 0.012 0.16

Next, samples (working examples 9 to 11) were produced by performingedge surface grinding under the production conditions of the workingexample 1 except that a resin bond grindstone having a differentgrindstone hardness was used. It should be noted that the lower thegrindstone hardness is, the smaller the cylindricity is. Thecylindricity was calculated as shown in FIGS. 4 and 5. That is, outlineswere obtained at the central position of the side wall surface in thesubstrate thickness direction and the positions spaced apart upward anddownward by 100 μm from the central position, the radii of inscribedcircles of the three outlines were derived, the difference between themaximum value and the minimum value of the radii of the inscribedcircles of the three outlines was derived, and the radius difference wasused as the cylindricity of the side wall surface. All measurements wereperformed using a roundness/cylindrical shape measurement device.

A magnetic layer and the like were formed on the magnetic-disk glasssubstrates of the working examples 9 to 11, and magnetic disks wereproduced. Each of the magnetic disks was incorporated in a 2.5-inch typeHDD having a disk rotation rate of 7200 rpm together with a DFH head,and after magnetic signals were recorded at a track density of 500 kTPI,servo signal reading testing was performed in a region of a radiusposition of 30.4 mm to a radius position of 31.4 mm.

[Evaluation Criterion]

The number of servo signal reading errors in the HDD was evaluated.Table 5 shows the results. The magnetic disks in which the error countis 30 or less are acceptable for practical purposes.

TABLE 5 Cylindricity (μm) Error count Work. Ex. 9 6 25 Work. Ex. 10 5 15Work. Ex. 11 3 12

It was confirmed from the evaluation results in Table 5 that althoughthe comparative example 9 was acceptable for practical purposes, therewere a larger number of servo signal reading errors in the case wherethe cylindricity exceeded 5 μm (working example 9) compared to the casewhere the cylindricity was 5 μm or less (working examples 10 and 11).

Next, the above-described processing conditions were changed asappropriate to produce two types of glass substrate having a substratethickness of 0.5 mm (the length of the side wall surface on the outercircumferential side was 0.3 mm) (comparative example 1A and workingexample 1A, respectively). The working example 1A and the comparativeexample 1A were produced such that the roundness and shape evaluationvalue of the working example 1A and the comparative example 1A were thesame as those of the working example 1 and the comparative example 1.When flutter was evaluated for these examples and improvement widthsresulting from a reduction in the shape evaluation value were compared,the improvement width from the comparative example 1A to the workingexample 1A was greater than the improvement width from the comparativeexample 1 to the working example 1. Therefore, it was confirmed that thepresent invention had a better effect on a thin glass substrate having asubstrate thickness of 0.5 mm or less, in particular.

While the magnetic-disk glass substrate according to the presentinvention has been described in detail, the present invention is notlimited to the above-described embodiment, and it will be appreciatedthat various improvements and modifications can be made withoutdeparting from the gist of the present invention.

What is claimed is:
 1. An annular substrate to be polished formanufacturing a magnetic-disk substrate having a circular hole at acenter, the annular substrate comprising: a pair of main surfaces; andan edge surface, the edge surface having a side wall surface andchamfered surfaces interposed between the side wall surface and the mainsurfaces, a roundness of the edge surface on an outer circumferentialside being 1.5 μm or less, and when outlines in a circumferentialdirection are respectively obtained at two positions spaced apart by 200μm in a substrate thickness direction on the side wall surface on theouter circumferential side, and a midpoint between centers of two leastsquare circles respectively derived from the outlines is given as amidpoint A, and when outlines in the circumferential direction arerespectively obtained at positions that are located at centers of thetwo chamfered surfaces on the outer circumferential side in thesubstrate thickness direction, and among centers of least square circlesderived from the outlines, a center derived from one chamfered surfaceis given as a center B, and a center derived from the other chamferedsurface is given as a center C, a sum of a distance between the midpointA and the center B and a distance between the midpoint A and the centerC being 1 μm or less, a substrate thickness of the annular substratebeing 0.635 mm or less.
 2. The annular substrate according to claim 1,wherein the sum is 0.5 μm or less.
 3. The annular substrate according toclaim 1, wherein in a case where, with regard to a surface roughness ofthe side wall surface on the outer circumferential side, a maximumheight in the substrate thickness direction is Rz(t) and a maximumheight in the circumferential direction is Rz(c), Rz(t)/Rz(c) is 1.2 orless.
 4. The annular substrate according to claim 1, wherein when ameasurement point is provided every 30 degrees in the circumferentialdirection, referenced on the center of the annular substrate, and aradius of curvature of a shape of a portion between the side wallsurface and the chamfered surface on the outer circumferential side atthe measurement point is derived, a difference in the radius ofcurvature between adjacent measurement points is 0.01 mm or less.
 5. Theannular substrate according to claim 1, wherein outlines of the sidewall surface in the circumferential direction are respectively obtainedat a plurality of different positions in the substrate thicknessdirection, the different positions include at least three positionsspaced apart by 100 μm in the substrate thickness direction on the sidewall surface on the outer circumferential side, an inscribed circle anda circumscribed circle of each outline are obtained, and a difference inthe radius between a smallest inscribed circle and a largestcircumscribed circle is 5 μm or less.
 6. The annular substrate accordingto claim 1, wherein a substrate thickness of the annular substrate is0.5 mm or less.
 7. A method for manufacturing a magnetic-disk substrate,the method comprising: polishing at least the main surfaces of theannular substrate according to claim
 1. 8. A method for manufacturing amagnetic disk, the method comprising: forming at least a magnetic layeron a main surface of the magnetic-disk substrate obtained from themethod according to claim
 7. 9. A magnetic-disk substrate having acircular hole at a center, the magnetic-disk substrate comprising: apair of main surfaces; and an edge surface, the edge surface having aside wall surface and chamfered surfaces interposed between the sidewall surface and the main surfaces, a roundness of the edge surface onan outer circumferential side being 1.5 μm or less, and when outlines ina circumferential direction are respectively obtained at two positionsspaced apart by 200 μm in a substrate thickness direction on the sidewall surface on the outer circumferential side, and a midpoint betweencenters of two least square circles respectively derived from theoutlines is given as a midpoint A, and when outlines in thecircumferential direction are respectively obtained at positions thatare located at centers of the two chamfered surfaces on the outercircumferential side in the substrate thickness direction, and amongcenters of least square circles derived from the outlines, a centerderived from one chamfered surface is given as a center B, and a centerderived from the other chamfered surface is given as a center C, a sumof a distance between the midpoint A and the center B and a distancebetween the midpoint A and the center C being 1 μm or less, a substratethickness of the magnetic-disk substrate being 0.635 mm or less.
 10. Themagnetic-disk substrate according to claim 9, wherein the sum is 0.5 μmor less.
 11. The magnetic-disk substrate according to claim 9, whereinin a case where, with regard to a surface roughness of the side wallsurface on the outer circumferential side, a maximum height in thesubstrate thickness direction is Rz(t) and a maximum height in thecircumferential direction is Rz(c), Rz(t)/Rz(c) is 1.2 or less.
 12. Themagnetic-disk substrate according to claim 9, wherein when a measurementpoint is provided every 30 degrees in the circumferential direction,referenced on the center of the magnetic-disk substrate, and a radius ofcurvature of a shape of a portion between the side wall surface and thechamfered surface on the outer circumferential side at the measurementpoint is derived, a difference in the radius of curvature betweenadjacent measurement points is 0.01 mm or less.
 13. The magnetic-disksubstrate according to claim 9, wherein outlines of the side wallsurface in the circumferential direction are respectively obtained at aplurality of different positions in the substrate thickness direction,the different positions include at least three positions spaced apart by100 μm in the substrate thickness direction on the side wall surface onthe outer circumferential side, an inscribed circle and a circumscribedcircle of each outline are obtained, and a difference in the radiusbetween a smallest inscribed circle and a largest circumscribed circleis 5 μm or less.
 14. The magnetic-disk substrate according to claim 9,wherein a substrate thickness of the magnetic-disk substrate is 0.5 mmor less.
 15. A magnetic disk in which at least a magnetic layer isformed on a main surface of the magnetic-disk substrate according toclaim
 14. 16. A hard disk drive comprising: the magnetic disk accordingto claim
 15. 17. A hard disk drive comprising: the magnetic-disksubstrate according to claim
 14. 18. A magnetic disk in which at least amagnetic layer is formed on a main surface of the magnetic-disksubstrate according to claim
 9. 19. A hard disk drive comprising: themagnetic disk according to claim
 18. 20. A hard disk drive comprising:the magnetic-disk substrate according to claim 9.